Mobile certificate distribution in a PKI

ABSTRACT

A method of providing certificate issuance and revocation checks involving mobile devices in a mobile ad-hoc network (MANET). The wireless devices communicate with each other via Bluetooth wireless technology in the MANET, with an access point (AP) to provide connectivity to the Internet. A Certificate authority (CA) distributes certificates and certification revocation lists (CRLs) to the devices via the access point (AP). Each group of devices has the name of the group associated with the certificate and signed by the CA. A device that is out of the radio range of the access point may still connect to the CA to validate a certificate or download the appropriate CRL by having all the devices participate in the MANET.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/270,080 filed on Oct. 10, 2011, which is a continuation of U.S.patent application Ser. No. 12/852,308 filed Aug. 6, 2010 and issued asU.S. Pat. No. 8,064,879 on Nov. 22, 2011, which is a continuation ofU.S. patent application Ser. No. 12/071,576 filed on Feb. 22, 2008 andissued as U.S. Pat. No. 7,787,865 on Aug. 31, 2010, which is acontinuation of U.S. patent application Ser. No. 10/634,881 filed onAug. 6, 2003 and issued as U.S. Pat. No. 7,356,329 on Apr. 8, 2008,which is a continuation of PCT International Application No.PCT/CA02/00130 filed on Feb. 5, 2002 which claims priority from U.S.Provisional Application No. 60/266,481 filed on Feb. 6, 2001, all ofwhich are hereby incorporated by reference.

FIELD OF INVENTION

The present invention relates to the field of cryptography and inparticular to mobile devices in public key infrastructures.

BACKGROUND OF THE INVENTION

The use of a Certificate Authority (CA) to distribute a public key of apublic key/private key pair is well known. A CA distributes public keysby creating and issuing certificates, documents which bind identifyinginformation about a user with the user's public key. Certificates alsotypically have information about the time period for which they arevalid. The public key, the identifying information, the lifetimeinformation, and other information such as a serial number are signedwith the CA's private key. The lifetime information, the identifyinginformation, the user's public key, and the signature together form thecertificate. Once a user has been issued a certificate, it can be usedby anyone who requires his public key; the user can present thecertificate to another party or the certificate can be stored in adatabase and retrieved by the party requiring the certificate. Providedthat the other users on the system have valid copies of the CA's publickey, a user's public key can be validated by verifying the signature onthe certificate with the CA's public key.

Several revocation methods exist to mitigate against the compromise ofthe private key of a public key/private key pair and to allow a user'sauthority to use a public key/private key pair to be taken away. Onerevocation method allows a certificate to be revoked. The CA may publisha certificate revocation list (CRL) containing the serial numbers of therevoked certificates and signed with the CA's private key; thevalidating party then downloads the CRL and verifies that a certificateis not in the CRL when validating the public key. Typically a CRL has acertain validity period and periodically new CRLs are generated. Anotherrevocation method is to use an online certificate status protocol (OCSP)where the validating party communicates with a trusted online entitythat confirms that a certificate has not been revoked. Yet a thirdrevocation method is to use short-lived certificates. With thisapproach, a CA frequently issues a certificate with a short lifetime. Auser retrieves his certificate from the CA after it is generated andpresents the latest short-lived certificate to the validating party. Ifcertification is to be revoked, the CA does not issue furthershort-lived certificates. Since the current certificate is short-lived,it will soon expire and it will no longer be possible to validate thepublic key using the certificate. The short-lived certificate makes itpossible for the validating party to accept a certificate with nofurther checking.

Current revocation methods are not well suited to an environment whereneither the presenter of a certificate nor the validating party has aconstant connection to the CA or another online entity. An example ofsuch an environment would be two wireless devices communicating solelywith one another using Bluetooth wireless technology. For example, ifCRLs are used, a device that needs to validate a certificate may beunable to download the appropriate CRL from the CA because it lacks aconnection to the Internet. OCSP requires an online check that may notbe possible. Short-lived certificates are promising in this environmentin that they do not require an online check at the time the certificateis validated, but fetching a new short-lived certificate presents aproblem when there is no connection to the CA.

It should be noted that checks of certificate revocation are notcurrently widely deployed in the wired internet. Server certificatessent to clients have been widely deployed, but client certificates sentto servers have not. As a web server is typically in a secure locationand typically uses a costly secure hardware token for private keystorage and signing, there is currently a certain degree of built-insecurity that lessens the need to check for certificate revocation.However for the scenario of one low-cost wireless device presenting acertificate to another low-cost wireless device, the ability to checkfor certificate revocation becomes more important. In this scenario, itmay be too costly to use a secure hardware token and there is also lessphysical security because the devices may not be in a secure location.

An object of the present invention is to obviate or mitigate the abovedisadvantages and provide a method and apparatus to facilitatecertificate revocation checks between two wireless devices where neitherhas a constant online connection.

SUMMARY OF THE INVENTION

In general terms the present invention provides a method of distributingcertificates to mobile devices by establishing a mobile ad-hoc network(MANET) between a plurality of mobile devices at a predetermined timeand distributing a certificate through said mobile ad-hoc network to arespective one of said mobile devices.

In a further aspect the present invention provides a method ofdistributing certificates in a mobile ad-hoc network having an accesspoint to provide a connection to a communication network and a pluralityof mobile devices to be connected to said communication network throughsaid access point, said method comprising the steps of retrieving andstoring at said access point certificates associated with respectiveones of said devices and forwarding said certificates through saidmobile ad-hoc network to said respective device.

In a still further aspect there is provided a method of distributingcertificates within a mobile ad-hoc network wherein an online entityassociated with a device is responsible for both distributing thedevice's certificate and for fetching other certificates needed to allowvalidation by another device in said network.

In a yet further aspect there is provided a method of securely setting atime source in a first device from a second device comprising the stepsof: establishing a shared secret between the two devices usingcertificates; storing the shared secret in a non-volatile memory; afirst of said devices authenticating a second of said devices using theshared secret; and transferring the time from the second device to thefirst device.

The invention also provides a method of validating wherein a certificatepresented to a first device by a second device is used for thevalidation if the second device's certificate has not expired andwherein the first device uses the validation certificate fetched basedupon a pointer presented by the second device if the second device'scertificate has expired.

A still further aspect of the invention provides a method ofdistributing certificates wherein a first device cannot retrieve acertificate at a first time because there is no connectivity to theinternet comprising the steps of: requesting assistance of other devicesif the certificate has still not been received by a second time; havinga second device of the other devices request the certificate on behalfof the first device when the second device has connectivity to theinternet; having the second device reestablish communication with thefirst device; and sending the certificate from the second device to thefirst device.

In a preferred embodiment there is provided a method of providingcertificate issuance and revocation checks involving mobile devices in amobile ad-hoc network (MANET). The wireless devices communicate witheach other via a wireless protocol, such as Bluetooth in the MANET, withan access point (AP) to provide connectivity to the Internet. ACertificate authority (CA) distributes certificates and certificationrevocation lists (CRLs) to the devices via the access point (AP). Eachgroup of devices has the name of the group associated with thecertificate and signed by the CA. A device that is out of the radiorange of the access point may still connect to the CA to validate acertificate or download the appropriate CRL by having all the devicesparticipate in the MANET.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the preferred embodiments of the inventionwill become more apparent in the following detailed description in whichreference is made to the appended drawings wherein:

FIG. 1 shows a number of wireless devices communication with each otherin an ad-hoc network;

FIG. 2 shows the timeline for a method for distribution of short-livedcertificates;

FIG. 3 shows a number of wireless devices communication with one anotherand authenticating each other through different certificate authorities;

FIG. 4 shows an example of a method by which the time in a portablelow-cost wireless device is securely set;

FIG. 5 shows the use of a pointer to a certificate in a MANET; and

FIGS. 6A and 6B show two examples of a method by which a non-connecteddevice that is not part of a specific ad-hoc group can receive itscertificate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is first made to FIG. 1, which shows wireless devices, 100,110, 120, 130, 140, and 150 in communication with one another via radiosusing a wireless protocol, such as the Bluetooth radio specification orone of the IEEE 802.11 physical layer specifications. The devices 100,110,120,130, 140 and 150 connect to a communication network such as theInternet 10 through one of the nodes identified as an access Point (AP)100. AP 100 is typically a fixed device with an AC power supply and thusmay be assumed to have a constant power supply. The nodes 110, 120, 130,140, and 150 have no direct connectivity to the internet 10; they areassumed to be mobile devices and having limited power sources such asbatteries. The radio in each wireless device is also assumed to have alimited range. In FIG. 1, the range of AP 100 is represented by circle107 having radius 104. Although, RF propagation is highly variable anddependant on the environment, it is assumed that AP 100 and any wirelessdevice within circle 107 can receive the transmissions of the othernodes within that circle. Similarly it is assumed that AP 100 and anywireless device outside of circle 107 cannot receive the other'stransmissions. Nodes 110, 120, 130, 140, and 150 similarly are assumedto have ranges represented by circles 117, 127, 137, 147, and 157,respectively, and having radii 110, 120, 130, 140, and 150,respectively.

Connectivity, between the nodes normally out of range of one another canbe obtained by implementation of a routing protocol between the nodes.This connectivity permits multiple devices to be interconnected and havebasis to establish empheral or short term networks of collections ofmobile devices. These collections of devices are typically referred toas mobile ad-hoc networks or MANET's. The Mobile Ad-hoc Networks (MANET)working group of the IETF is currently developing routing protocols toenable devices such as 110, 120, 130, 140, and 150 to form mobile ad-hocnetworks (MANETs) through which they may communicate with one another.As shown in FIG. 1, devices 110 and 130 cannot communicate directly withone another. If devices 110, 120, and 130 implement a MANET routingprotocol, device 120 may serve as a router and enable devices 110 and130 to exchange packets with one another even though they are out ofradio range of one another. A MANET may have connectivity to theinternet 10 through access points such as 100. In FIG. 1, a MANETenables device 150 to send a packet to a node in the internet routed viathe following nodes in order: 140, 130, 120, 110, and 100. In a MANET,nodes are free to move around and the routing protocol must adapt. Forexample, if nodes 140 and 120 changed places with one another a MANETrouting protocol would adapt such that device 150 could still send apacket to a node in the internet, but routed via the following nodes inorder: 120, 130, 140, 110, and 100. MANETs are described in furtherdetail in RFC2501. A suitable MANET routing protocol is the Ad-hocOn-Demand Distance Vector (AODV) Routing protocol described indraft-ietf dsr-07.txt. This reference, the contents of which areincorporated herein by reference, is available athttp://www.ietf.org/internet-drafts/draft-ietf-manet-aodv-07.txt.Another suitable MANET routing protocol is the Dynamic Source RoutingProtocol for Mobile Ad-hoc Networks described in draft-ietfmanet-dsr-04.txt. This reference, the contents of which are incorporatedherein by reference, is available athttp://www.ietforg/internet-drafts/draft-ietf_manet-dsr-04.txt.

Devices 100, 110, 120, 130, 140, and 150 all are part of a group ofdevices which can form an ad-hoc network with one another to provide anad-hoc group indicated by chain dot line 160. Devices in the group 160have certificates to enable them to authenticate each other and confirmthey are part of a privileged group. Since devices 110, 120, 130, 140,and 150 have limited power sources, they may wish to avoid participationin ad-hoc networks with other devices that are not in the group. Devices110, 120, 130, 140, and 150 may have the capability to participate in anad-hoc network with one another, but if a device has no application thatwould make use of an ad-hoc network at a particular time, it may declineto participate because of the drain on its power source thatparticipation would cause.

The Certificate Authority (CA) 50 is responsible for issuing short-livedcertificates to the devices and for sending the short-lived certificatesto the devices upon request. Since the validity of a certificate dependsupon the current time, devices 100, 110, 120, 130, 140, and 150 all haveaccurate time sources that are always available for validatingcertificates. In a preferred embodiment, devices 100, 110, 120, 130, 140and 150 are distinguished as members of an ad-hoc group by giving thegroup a name, making all of the devices aware of the group name, andincluding the name in the certificates signed by the CA. When thedevices check a certificate for the purpose of authenticating a fellowad-hoc group member they will verify that the certificate contains thegroup name.

In an alternate embodiment, the root key of the members' CA defines themembership of a group; all devices having certificates signed by thesame CA's private key comprise a group.

Although the functions of issuing short-lived certificates to thedevices and for sending the short-lived certificates to the devices areshown as being part of a traditional CA, it should be noted thatequivalent functionality could be provided with the addition of an OCSPclient. In this case, a CA would not issue short-lived certificates; thecertificates issued by a CA would have a long validity period. The CAwould act as an OCSP responder and upon request from the OCSP clientwould indicate whether or not a public key/private key pair is valid.Rather than having a CA issue short-lived certificates, the OCSP clientwould issue precomputed OCSP responses having the validity period asdescribed for a short-lived certificate. The OCSP client would issue aprecomputed OCSP response only if the CA indicates that the publickey/private key pair is valid. Rather than fetching a short-livedcertificate from a CA, a wireless device could fetch a precomputed OCSPresponse from an OCSP client. The full certificate issued by the CAcould optionally be delivered to the wireless device along with theprecomputed OCSP response. This possible use of a precomputed OCSPresponse rather than a short-lived certificate applies to all figuresand descriptions.

If devices 100, 110, 120, 130, 140, and 150 were always in theconfiguration shown in FIG. 1 and were always participating in an ad-hocnetwork, retrieval of short-lived certificates would be a simple matteras there is always a route to CA through the internet. However, themobility of the devices presents difficulties to the retrieval ofshort-lived certificates. For example, if device 110 were to move out ofrange of all of the other devices or if it were to decline toparticipate in the ad-hoc network, devices 120, 130, 140, and 150 wouldall be unable to fetch their short-lived certificates from CA becausethey are all out of range of AP. Prolonged lack of ad-hoc activity amongall devices followed by a need to establish an ad-hoc network wouldresult in failure if the short lived certificates expired and thedevices needing to establish the ad-hoc network were all out of range ofAP.

In order to mitigate the problems associated with the lack of a route tothe CA when a device needs to fetch its short-lived certificate, alldevices are required to participate in a MANET at the same time atperiodic predetermined times, provided that other devices are withinrange. While the devices are participating in the MANET, they fetchtheir short-lived certificates. This time period during which a MANET isestablished for the purpose of updating certificates will be referred toas a MANET Establishment Period or MEP. By having all members active inthe MANET at the same time, the probability that there will be a routeto the CA is maximized. Preferably, the times for which the short-livedcertificates are valid are related to the periodic predetermined timesof the MEP. For example, the beginning of a short-lived certificate'svalidity period may correspond to the beginning time of a MEP.Similarly, the ending of a short-lived certificate's validity period maycorrespond to the end time of a MEP. Preferably, the validity period ofa short-lived certificate will have a substantial overlap withsubsequently issued short-lived certificates such that a device that hasno route to the Internet during one MEP may continue using theshort-lived certificate at least until a subsequent MEP as illustratedschematically in FIG. 2.

Referring to FIG. 2, time bars 260, 270, 280, and 290, each representseparate time periods for which groups of certificates issued by CA 50are valid. For example, prior to time 200, CA 50 issues a first group ofshort-lived certificates, one for each device 100, 110, 120, 130, 140,and 150. Each short-lived certificate in the group is valid beginning atthe time period beginning at time 200 and ending at time 227. Similarlythere are groups of certificates valid during time bars 270, 280, and290. For exemplary purposes only it will be assumed that these timeperiods are each 48 hours and 20 minutes long.

During the time period from 200 to 207, devices participate in a MANETfor the purpose of fetching short-lived certificates, provided the otherdevices are within range. The time period from 200 to 207 is a MEP. Thetime periods from 210 to 217, 220 to 227, 230 to 237, 240 to 247, and250 to 257 are all also MEPs. For purposes of discussion it can beassumed that the time between the start of each of these MEPs is 24hours; e.g. the time between 200 and 210 is 24 hours. Assuming thatthese time periods are 20 minutes in length, then times 204, 214, 224,234, 244, and 254 all represent midpoints in these time periods,dividing them into 10-minute halves.

The CA tracks which devices have received their updated short-livedcertificates. This is accomplished by having the devices sendacknowledgements to the CA when they have received their short-livedcertificates. In order to prevent an attacker from faking anacknowledgement, the acknowledgements include a signature of thesignature of the certificate, signed by the device's private key. The CAwould only update the status if the signature were good, determined by averification of the signature with the particular device's public key.

It can be assumed that at time 210, all of the devices have short-livedcertificates with validity period 260. At time 210, devices 100, 110,120, 130, 140, and 150 will all attempt to establish an ad-hoc network.Suppose that all devices except 140 and 150 are within range as shown inFIG. 1. Devices 140 and 150 are both out of range of any other deviceand have no route to CA 50. Devices 100, 110, 120, and 130 successfullyjoin an ad-hoc network. At time 214 the devices 100, 110, 120, and 130,having successfully joined the ad-hoc network each send a message to CA50 requesting an updated certificate, CA 50 upon receiving such amessage sends a message in reply containing the requesting device'scertificate with validity period represented by the time bar 270. Wheneach device receives its certificate, it verifies the signature in thecertificate with CA 50's public key; if the certificate is valid, thedevice will sign the signature on the certificate with its private keyand send this signature to CA 50 as the acknowledgement. Upon receivingthe acknowledgement, CA 50 will check the validity of the signatureusing the device's public key and if it is verified to be the device'ssignature, the CA will mark the device as having received itscertificate. Shortly after time 214, the CA will have marked 100, 110,120, and 130 as having received their certificates for time period 270.Since devices 140 and 150 are out-of-range, the CA will not have markedthem as having received their certificates for time period 270. At time217, the devices are allowed to discontinue the ad-hoc network that wasestablished at time 210.

After time 217, devices 140 and 150 are aware that they were unable tocommunicate with the CA 50 to fetch their short-lived certificates fortime period 270. Because devices 140 and 150 do not have their mostrecent certificates, between time 217 and 220, devices 140 and 150 willcontinue to attempt to retrieve their certificates. As time progresseswithout a device having retrieved its short-lived certificate, it maytake progressively more desperate measures to retrieve its certificate.After time 217, devices 140 and 150, if they are within range of anaccess point such as AP 100, should immediately connect in order tofetch their short-lived certificates from CA 50. Since it would consumemore power from a device's power source, a progressively more desperatemeasure would be to participate in ad-hoc networks with other deviceswhenever possible; at a first time period after time 217 but before time220, devices would take this progressively more desperate measure.Devices such as 140 and 150 may be multi-mode devices, including thecapability to use a cellular packet data system such as cdma2000 packetdata or GPRS packet data in addition to the short-range radiotechnology. The coverage of a cellular packet data system will typicallybe much greater than that of short-range radio technology. However,short-range radio technology typically uses an unlicensed band and iseither free or very low-cost. For a dual-mode device, a progressivelymore desperate, and more expensive, measure to retrieve a short-livedcertificate involves establishing a packet data call through thecellular packet data system for the purpose of retrieving a short-livedcertificate. At a second time period after the first time period butbefore time 220, dual-mode devices would take this even more desperatemeasure to retrieve a short-lived certificate.

At time 220, devices 100, 110, 120, 130, 140, and 150 make anotherperiodic attempt to establish a MANET for the purpose of fetchingshort-lived certificates. The establishment of the MANET during the timeperiod from 220 to 224 represents a last opportunity of certificateretrieval for a device unable to fetch its short-lived certificate sincetime 210. After time 224, the CA 50 takes the progressively moredesperate measure of distributing such a device's certificate to otherdevices that have established a route to the CA. Assume that by time 220device 140 has returned to its position as shown in FIG. 1, that device150 remains out of range of any other device, and that devices 100, 110,120, and 130 are as pictured in FIG. 1. At time 220, device 140 sends amessage to CA 50 requesting its certificate. Upon receiving the message,CA 50 sends to device 140 its short-lived certificate. Preferably theshort-lived certificate would be the latest—the one with validity period280. Upon receiving the certificate, device 140 sends acknowledgement toCA 50. Upon receiving the acknowledgement, the CA marks device 140 ashaving received an up-to-date certificate.

After time 224, devices that are able to will send messages to CA 50requesting up-to-date short-lived certificates. CA 50 will respond toeach request by sending the requesting device's short-lived certificate.In addition, CA 50 will also send to each device requesting its ownup-to-date short-lived certificate the short-lived certificate ofdevices that it has not marked as having a certificate that expires atthe end of validity period 270 or later. Thus any device that was notable to retrieve its certificate since the previous MEP will have itscertificate delivered to devices that are able to retrieve theircertificates during the current MEP. At time 224 devices 110, 120, and130 send requests to CA 50 for their most recent short-livedcertificate. CA 50 responds to each with the device's certificate andalso the certificate of device 150 since device 150 is not marked ashaving a certificate that expires at the end of validity period 270 orlater. Since device 140 already received its up-to-date short-livedcertificate between times 220 and 224, it need not receive thecertificate again after time 224. Nevertheless, device 140 will send arequest message to CA 50. CA 50 will respond with a short-livedcertificate of device 150 (i.e. the short-lived certificate of devicesthat it has not marked as having a certificate that expires at the endof validity period 270 or later). The CA could also include theup-to-date short-lived certificate of device 140, but for bandwidthsavings, it preferably would not since it had already been delivered.

Subsequently, during the time between 227 and 230, device 150 encountersdevice 130 and 120 and wishes to establish an ad-hoc network, but all ofthese devices are out-of-range of AP 100 or any other AP and so cannotestablish a route to CA 50. Accordingly, either device 120 or 130 willbe able to authenticate device 150, using its copy of a currently validcertificate for device 150. When device 150 interacts with either ofthese devices, it preferably receives and stores a copy of itsup-to-date certificate from the other device. In this way, device 150can present its certificate if it subsequently interacts with anotherdevice that was unable to receive the certificate during the time periodfrom 224 to 227. It should be noted that even though device 150 hasalready received its certificate via another device, it should send anacknowledgement of this to CA 50 at any subsequent opportunity prior totime 234. This may be done via the establishment of an ad-hoc networkwith a route to CA or alternatively by interaction with an AP. Theacknowledgement should include a signature of the certificate signed by150's private key. If the CA receives this acknowledgement prior to time234, it can avoid distributing 150's new certificate to other devicesfrom time 234 to 237. After device 150 retrieves its certificate fromthe other device, the other device need not store the certificate anylonger; it may free the memory resource. The device 150 also sends thesame acknowledgement as sent to CA 50 to a device storing itscertificate and the device would verify the signature in theacknowledgement before deleting the certificate from memory.

In the event that a large number of devices are unable to communicatewith a CA for a long period of time, the number of other devices'certificates that the CA would be required to send to a device (and thenumber the device is required to store in its memory) could becomelarge. This could happen, for example, if a large number of devices wereturned off. In this case, some of the certificates could be excluded ifeither bandwidth or memory is a limitation. The CA could selectivelyexclude some devices' certificates from the ones sent to a particulardevice if bandwidth to the device is a problem. If memory in a device isthe limitation, the device could choose not to store some of thecertificates sent by the CA. If it becomes necessary to not sendcertificates or to not store certificates due to bandwidth or memorylimitations, preference is preferably given to certificates of thosedevices that have communicated with the CA most recently. Certificatesof the devices most recently in communication with the CA should be sentand stored rather than the certificates of the devices least recently incommunication with the CA.

As described above, FIG. 1 and FIG. 2 show the update of short-livedcertificates based upon establishing MANETs during MEPs atpre-determined times. Alternately the times of the MEPs could bedetermined dynamically. The devices could keep a record of when theyencounter other devices in their ad-hoc group, regardless of whether ornot they actually join an ad-hoc network. This could be, for example,via the bluetooth service discovery process for devices using bluetoothwireless technology. When a device interacts with the CA, for examplewhen retrieving certificate information during a MEP, it sends thisinformation to the CA. The CA would then have this information for alldevices in an ad-hoc group. The CA could analyze this information todetermine times when many devices were in proximity of others with aroute to the internet and attempt to predict good times for the devicesto establish ad-hoc networks for updating certificates. The CA could,for example, make a measurement of the number of devices in range of oneanother during various time periods of the same duration and choose forthe MEP the time period where there are the most devices in range of oneanother. The dynamically determined MEP time period could be distributedto the devices in a certificate using a pre-determined relationship withthe validity times of a certificate. Alternately, separate fields of thecertificate could be dedicated to define the MEP time period.

In the above example, one might assume that the certificates beingdistributed had all certificate information one might expect in acertificate. It should be noted that for short-lived certificates mostinformation remains the same when the certificate is updated. Since alist of certificates may be stored in multiple devices, saving memory isa concern and it may be preferable to use a short form of theinformation when updating short-lived certificates. The full certificatemay be stored in the device, but the CA may distribute only theinformation that has changed—the timing information and the signature.The other information may be implicitly covered by the signature is thehash and signature from the CA. However, when presenting a certificate,a device would present the full certificate.

In the above example, all devices had the same CA and all had embeddedroot keys of the same CA. In some situations it may be desirable to havedevices with different CAs authenticate each other, as shown in FIG. 3.

Wireless devices 300, 310, 320, 340, and 350 contain radios that cancommunicate with one another using a common air interface such as theBluetooth radio specification or one of the IEEE 802.11 physical layerspecifications. In addition to a wireless interface, the AP 300 hasconnectivity to the internet 10. Nodes 110, 120, 140, and 150 have nodirect connectivity to the internet. The radio in each wireless devicehas a limited range. The range of AP 300 is represented by circle 307having radius 304. Nodes 310 (DI), 320 (D2), 340 (D3), and 350 (D4)similarly are assumed to have ranges represented by circles 317, 327,347, and 357, respectively, having radii 310, 320, 340, and 350,respectively.

D2 is only within range of DI. D1 is within range of AP and D2. AP isonly within range of DI. D3 and D4 are only within range of each other.D2 has connectivity to the internet 10 routed through D1 via an ad-hocnetwork. Devices DI, D2, D3, and D4 each has for its CA 360 (CA1), 370(CA2), 380 (CA3), and 390 (CA4), respectively, and each has an embeddedroot certificate of its CA. In addition CA1 has generated a certificateincluding CA2's public key and CA3 has generated a certificate includingCA4's public key. Also CA5 has generated certificates including publickeys for CA1 and for CA3. In addition to the root key for CA2, D2 alsohas embedded the root key of CA5. Also, in addition to the root key forCA4, D4 also has embedded the root key of CA5.

In a first example, an AP such as AP 300 may fetch and storecertificates on behalf of the devices with which it can exchangepackets. It should be noted that the same would apply if D2 and D1 bothwere within range of AP 300 and that the routing function of DI were notneeded by D2. When new short-lived certificates are issued, AP 300fetches them from the appropriate CA. When a new short-lived certificateis issued for D1, AP fetches it from CA1. When a new short-livedcertificate is issued for D2, AP fetches it from CA2. When a device withwhich AP 300 can exchange packets needs to authenticate another node, itfetches the node's certificate from AP 300. In addition, AP 300 queriesthe devices with which it can exchange packets for their embedded rootkeys. Based upon the devices present with which AP 300 can exchangepackets and the root keys of the devices, AP 300 will also fetchcertificates to enable the devices to authenticate each other. BecauseD1 does not have the root key of CA2 embedded, AP fetches thecertificate signed by CA1 including CA2's public key. When DI needs toauthenticate D2, it will send a request to AP 300 indicating that it isauthenticating D2. AP 300 will send to DI D2's certificate and thecertificate signed by CA1 including CA2's public key; this will enableD1 to authenticate D2. Because D2 does not have the root key of CA1 butdoes have the root key of CA5 embedded, AP fetches the certificatesigned by CA5 including CA1's public key. When D2 needs to authenticateDI, it will send a request to AP 300 indicating that it isauthenticating D1. AP 300 will send to D2 DI's certificate and thecertificate signed by CA5 including CA1's public key; this will enableD2 to authenticate D1.

In a second example, a device's own CA will be responsible for bothdistributing the device's short-lived certificate and for fetching anyother certificates needed to allow the device's short-lived certificateto be validated. Each device will be responsible for collecting theembedded root keys of other devices with which it comes in contact withvia either direct radio contact or via presence in the same ad-hocnetwork. When fetching its short-lived certificates from the CA a devicewill report these other devices and their root certificates to its CA.Based upon the root certificates for a device, the CA will determineother certificates needed to allow that device to be authenticated andwill return those other certificates to the requesting device along withits short-lived certificate. When D3 and D4 encounter one another, D3receives D4's embedded root keys (CA4 and CA5); likewise D4 receivesD3's embedded root key (CA3). When D3 requests its short-livedcertificate from CA3, it reports D4's root keys (CA4 and CA5). To enableD4 to authenticate D3, CA3 sends to D3, along with its short-livedcertificate, the certificate signed by CA5 including CAS's public key.When D4 requests its short-lived certificate from CA4, it reports D3'sroot key (CA3). To enable D3 to authenticate D4, CA4 sends to D4, alongwith its short-lived certificate, the certificate signed by CA3including CA4's public key. Thus when the devices authenticate eachother, by sending the certificates received from their CAs, theappropriate information will be available.

In addition to having an AP such as AP 300 fetch short-livedcertificates from a CA on behalf of devices with which it can exchangepackets, CAs may sometimes send, “push”, short-lived certificatesdirectly to APs. For example, suppose that a certain device is withinrange of AP 300 most of the time, but not at the moment when thedevice's short-lived certificate is generated. The device's CA can keeptrack of the APs that the device is commonly within range of and pushthe short-lived certificates to all of these APs when the short-livedcertificate is generated. Thus when the device enters the vicinity ofthe AP such as AP 300, its short-lived certificate is immediatelyavailable and a round trip through the internet to the CA can beavoided.

Because a certificate is valid only for a certain time period, a devicemust have a correct time source to be able to validate a certificate.Methods are known for setting time in the Internet; see, for example,RFC 1305 and draft-ietf-stime-ntpauth-00.txt. This reference isavailable athttp://www.ietf.org/internet-drafts/draft-ietf-stime-ntpauth-00.txt.However, a low-cost wireless device in many cases will not be set to thecorrect time out-of-the-box since a battery or other power source is notlikely to be installed from the factory. It is desirable to be able tosecurely set the time in such an out-of-the-box device when power isapplied. Furthermore, a portable low-cost wireless device with the timecorrectly set is unlikely to have enough energy to maintain the correcttime when its power source is removed. Thus a device owner changing thebattery in such a device may compromise the device's ability to validatecertificates. Therefore, it is desirable to be able to quickly reacquirethe correct time when power is reapplied in such a circumstance.

Referring to FIG. 4, illustrates schematically a method of securelysetting the time in a portable low-cost wireless device. The wirelessdevices, 400, 410, 440, 450, and 460, communicate with one another viaradios using a common air interface, such as the Bluetooth radiospecification or one of the IEEE 802.11 physical layer specifications.In addition to a wireless interface, the AP 400 has connectivity to theinternet 10. Nodes 410, 440, 450, and 460 have no direct connectivity tothe internet. The radio in each wireless device has a limited range. Therange of AP 400 is represented by circle 407 having radius 404. Nodes410, 440, 450, and 460 similarly are assumed to have ranges representedby circles 417, 447, 457, and 467, respectively, having radii 414, 444,454, and 464, respectively. AP 400 and device 410 are within range ofone another. Device 450 is within range of both device 440 and device460. Devices 440, 450, and 460 all are without connectivity to theinternet. Device 410 has connectivity to the internet 10 via AP 400. TS490 is a secure and accurate time source. TS 490 has an accurate clockfor maintaining the current time. TS 490 also includes tamper-proofsigning ability with the private key of a public key/private key pairused for signing. Devices 410, 440, 450, and 460 all have embedded inmemory the public key associated with the private key in thetamper-proof hardware of TS 490. The memory used for storing the publickey survives the loss of power by the device such as occurs when thedevice has no battery.

For a first example, assume that device 410 is an out-of-the-box devicewhose battery has just been inserted. When the battery is inserted intodevice 410, circuitry in the device determines via well-known means thatit is a cold boot up and that the clock is not accurately set. Device410 establishes a connection with AP 400; this may include device 410presenting its certificate to AP 400. Device 410 must first set itsclock accurately and securely before trusting AP 400. Device 410 willgenerate a random challenge to send to TS 490. Device 410 will send amessage to AP 400 including the random challenge and information aboutTS 490, such as an internet address, which allows AP 400 to communicatewith TS 490. AP 400 sends the random challenge to TS 490. When TS 490receives the random challenge, it takes the current time from its clock,concatenates the random challenge to it, performs a hash function overthe time and challenge, signs the hash with its private key, and returnsthe time, challenge, and signature to AP 400 which then forwards it todevice 410. If the challenge returned matches the challenge that wassent and the round trip time is acceptably short, device 410 then checksthe signature using its embedded public key of TS 490. If the signatureis verified, device 410 will set its clock to the received time. Theaccuracy of the time is a function of the round trip time. With itsclock now set, device 410 can now validate a certificate from AP 400 orfrom any other device. If AP 400 and device 410 are exchangingcertificates, device 410 could send the challenge to AP 400 in the samemessage as device 410's certificate; AP 400 could then send itscertificate to device 410 in the same message as the time, challenge,and signature.

The above example shows a case where the device is within range of an APwhen it is first powered on. If device 400 had not been within range ofAP 400, it could have still used TS 490 to set the time via an ad-hocnetwork with one or more intermediate nodes routing packets to TS 490via AP 400. If device 400 were in a situation where there is no directconnectivity to the internet, it should take the first possibleopportunity to set the clock via an ad-hoc network. If device 400 is amulti-mode device, including the capability to use a cellular packetdata system, it could take a more desperate measure to set the time ifthere were an immediate need to validate a certificate. The device couldestablish a packet data call through the cellular packet data system forthe purpose of communicating with TS 490 to set the time.

For a second example, assume that device 450 is powered-on and that itsclock already has the correct time, having been set during a previousinteraction with TS 490 through AP 400. Also assume that device 460 isan out-of-the-box device whose battery has just been inserted. When thebattery is inserted into device 460, circuitry in the device determinesvia well-known means that it is a cold boot up and that the clock is notaccurately set. Since device 460 has no way of connecting to theinternet, it cannot immediately set its clock using TS 490. Assume thatdevice 450 and device 460 are owned and controlled by the same person.The person wishes to establish a trusted relationship between these twodevices and does this using well known means. A suitable example wouldbe by using the procedure described in the Bluetooth SecurityArchitecture paper. Once this trusted relationship is established,device 460 trusts device 450 and can use device 450 as a time source.When device 460 determines that it has established a trustedrelationship with device 450, it requests the time from device 450 usinga secure channel. Device 450 responds with the current time from itsclock. If device 460 receives the response within a preset time periodafter sending the request, it will set its clock to the time sent bydevice 450. Device 460 can then use its own time source to validatecertificates. Subsequently when device 460 is able to establish aconnection to the internet such as via AP 400, it may reset its clock asdescribed in the first example using TS 490; this would correct anyaccumulation of errors as the time is propagated from device to device.

In yet another example, assume that both device 440 and device 450 arepowered-on and that their clocks already have the correct time, havingbeen set during previous interactions with TS 490 through AP 400.Devices 440 and 450 have just been brought within radio range of oneanother. Although it is shown in FIG. 4 that devices 440 and 450 maycommunicate directly with one another, the following time-setting methodwould be applicable if devices 440 and 450 were merely in the same MANETand other devices routed packets between them. Devices 440 and 450 haveno shared secret between them, but each has a public/private key pairand up-to-date short-lived certificates and they have a trustedrelationship that can be determined via certificates and well-knownpublic key methods. The devices discover one another, exchangecertificates, and determine via public key methods that they have atrusted relationship. The devices then establish a shared secret. Eachdevice then stores the identity of the other, the shared secret, and anexpiration time in a non-volatile memory, i.e. a type of memory wherethe information will still be present when power is reapplied after adevice's source of power has been lost.

After a period of time, the owner of device 440 begins the process ofchanging the battery. The owner removes device 440's battery and theclock ceases to function. The owner then inserts a new battery and poweris restored to device 440. When the battery is inserted into device 440,circuitry in the device determines via well-known means that it is acold boot up and that the clock is not accurately set. Preferably device440 would set its clock via TS 490, but device 440 determines that thereis no path to the internet which can be used to set its clock via TS490. Device 440 discovers that it has a path to device 450 (eitherwithin radio range or routed through a MANET). Because device 440 has ashared secret with device 450 stored in non-volatile memory, itcommunicates with device 450 and uses the shared secret to authenticatedevice 450 via well-known means. After successfully authenticatingdevice 450, device 440 requests the time from device 450. Device 450responds with the current time from its clock. The communication betweendevices 440 and 450 is preferably encrypted. If device 440 receives theresponse within a preset time period after sending the request, it willset its clock to the time sent by device 450. Device 440 can then useits own time source to validate certificates. Subsequently when device440 is able to establish a connection to the internet such as via AP400, it may reset its clock as described in the first example using TS490. This would correct any accumulation of errors as the time ispropagated from device to device. The shared secret stored innon-volatile memory is intended to be a temporary measure used to assista nearby device in setting the time. A device without a power source andwhose clock has stopped could be vulnerable to an attacker trying toincorrectly set the time in the device if the attacker had the sharedsecret. The expiration time of the shared secret (to be observed bydevices with power sources and whose clocks are still accurate) shouldtherefore be set to a relatively short time. In this way, the risk ofthe shared secret being compromised is minimized. When a device with apower source and an accurate clock determines that the expiration timeof the shared secret has passed, it should destroy the shared secret inits memory.

Some devices with a very limited power source may benefit from theshort-lived certificate distribution methods described above. A securetoken used for authentication is an example of such a device. A securetoken is a small-sized device that might typically be carried on a keychain. The battery on a token would be expected to last a very long timeand would not typically be rechargeable. Such a token would typicallynot participate in an ad-hoc network because of the requirement topreserve the life of the power source. Such a token could be paired withanother device that the owner of the token carries with him such as aBluetooth PDA or cellular phone that does typically participate inad-hoc networks. The paired device would take the responsibility offetching short-lived certificates on behalf of the token and deliveringshort-lived certificates to the token when fetched.

It is known for a device to present to a validating party a pointer toits certificate rather than the certificate itself. According to the WAPstandard, the client may present a Uniform Resource Locator (URL), asthe pointer, to a certificate to a server rather than the certificateitself. Being a node on the internet, the server may fetch thecertificate using the URL. When the wireless link between the client andthe server is very slow in a WAP system, presenting an URL rather thanthe certificate itself can significantly speed up the certificatevalidation. The reason is that the URL is much shorter than thecertificate itself and thus takes less time to transmit over the airinterface. The saved time on the air interface will be smaller than theadditional time required for the server to fetch the certificate usingthe URL.

Referring to FIG. 5, in order to show the use of a pointer to acertificate in a MANET. Suppose that devices 500, 510, 520, and 530 areall involved in a MANET. Device 530 has connectivity to the internet byrouting packets through devices 520, 510, and 500. CA 550 is theCertificate Authority. Device 540 is shown as not being involved in theMANET, but is moving towards device 530. When device 540 is within rangeof device 530, it will attempt to join the ad-hoc network. Device 530will authenticate device 540 using its certificate. Device 540 has anURL that points to its short-lived certificate. CA 550 stores device540's certificate. The URL to the certificate points to CA 550; thiswould be, for example, by having CA 550's server name as part of theURL. In order to retrieve the certificate using the URL, device 530first retrieves the IP address of CA 550 using the internet Domain NameSystem (DNS). The request is routed through devices 520, 510, and 500.The IP address is then sent to device 530, routed through devices 500,510, and 520. Once the IP address is known, device 530 sends the requestto CA 550, routed through devices 520, 510, and 500. The certificate isthen sent from the CA 550 to device 530, routed through devices 500,510, and 520. Device 530 can then validate the certificate. The twotraversals through the MANET clearly add significant time to thevalidation; they also consume bandwidth in the MANET and drain thebatteries of devices 510, 520, and 530. One traversal could potentiallybe saved by including CA 550's IP address in the URL rather than itsname, but the traversal still required would still be expensive.Furthermore, the current technology typically used for MANETs has a highenough transmission speed between nodes that the time savings achievedby sending an URL from node 540 to 530 rather than a certificate isnegligible. In addition, a path to the CA for fetching a certificateusing an URL may not even be present. In most cases it is clearlypreferable for a device in an ad-hoc networking environment to present acertificate to another device rather than an URL.

As described with respect to the previous figures, however, whenshort-lived certificates are used and a device's short-lived certificateexpires before it has been able to fetch a new one, the device will notbe able to present a valid certificate to a validating device. In thissituation, it would be desirable for the device to be able to present apointer to a certificate rather than the certificate itself. Since thedevice has not yet been authenticated, it would not have the authorityto fetch the certificate from the CA itself since it has not yet beenpermitted to join the MANET; i.e. it is assumed that, prior toauthentication, intermediate nodes in the MANET would not route packetson behalf of the device. Even if the device were permitted to fetch thecertificate itself, this could require two more message exchanges thanpresenting an URL. For example, in the example of FIG. 5, a request fromdevice 540 for its updated certificate would be routed to the CA 550through devices 530, 520, 510, and 500. The response would then berouted through devices 500, 510, 520, and 530. By presenting an URL withan IP address included, the time required to send the certificate from530 to 540 and then back to 530 could be eliminated.

Preferably a device will present a short-lived certificate to avalidating party when the device has an up-to-date short-livedcertificate to send. When a device does not have an up-to-dateshort-lived certificate to send, it preferably presents a pointer, suchas an URL, to a validating party and then the validating party fetchesthe certificate using the pointer. Alternately, a device could alwayspresent both its latest short-lived certificate and a pointer to ashort-lived certificate to a validating party; the validating partycould then use the presented certificate if it is valid or fetch thelatest certificate using the presented pointer if the presentedcertificate is not valid.

Multiple possibilities exist for the format of the URL presented by adevice that uses short-lived certificates. One option is to have the URLalways remain the same. In this case, when the CA issues a newcertificate, it changes the certificate that is pointed to, that is, thecertificate that will be returned to a requesting device, but the URLpresented by the requesting device would remain the same. Another optionwould be to have a first part of the URL be fixed, and also to includetime as a second part of the URL presented to the CA and to have the CAreturn a certificate based upon the time in the URL. In this way, in thefuture it would be very easy to check if there were valid certificatesfor a device at various times in the past; this could be used to provethat a transaction took place.

Previous examples of methods of distributing short-lived certificatessuch as described with respect to FIG. 1 and FIG. 2 take advantage ofknown pre-defined groups of mobile devices to enable certificates to be“pushed” to devices with connectivity to the internet. For example, ifdevices A and B are part of a group and device A is disconnected fromthe internet for an extended period of time and its short-livedcertificate expires, after a certain point, the CA will recognize thatdevice A has not retrieved its certificate. The CA would then “push”device A's certificate to other devices in the group that do haveconnectivity to the internet. For example, the CA would “push”, or sendthe certificate to device B if device B has connectivity to the internetwhen A does not. Subsequently, if B interacts with A, it can send to Aits short-lived certificate. It would also be desirable for a devicewith no connection to the internet to be able participate in ad-hocnetworks and to receive its certificate even if it is not part of apre-defined group of devices.

As shown in FIGS. 6A and 6B, nevertheless a non-connected device that isnot part of a specific ad-hoc group can receive its certificate. At afirst time, device 640 attempts to retrieve its certificate, but it isunable to because there is no connectivity to the Internet. Until asecond time, device 640 continues to attempt to retrieve itscertificate. If by the second time, device 640 has been unable toretrieve its certificate, it will begin requesting assistance from otherdevices. For example, device 640 would send a request to all neighboringdevices within radio range (such as device 630). Furthermore, therequest can be broadcast to all devices participating within the sameMANET. For example, in the first configuration shown in FIG. 6A, device640 would broadcast the request to devices 630, 620, and 610 since theyare all members of the MANET. The request tells the other devices thatif they subsequently have a path to the internet that they are to fetchan up-to-date short-lived certificate on behalf of device 640. Uponreceiving the request that includes the identity of device 640 and anaddress of device 640's CA, a device will record the request in itslocal memory. For example, device 610 receives a request and stores itin its local memory. In the second configuration shown in FIG. 6B,assume that device 610, after having received the request from device640, moves out of range of the MANET, but within range of the AP. Sincedevice 610 had stored the request, and it now has a route to theinternet through device 600, it will fetch device 640's up-to-datecertificate from device 640's CA 650. Upon receiving the certificate,device 610 will store it in its local memory. Subsequently, if device610 joins the same MANET as device 640, it will send the certificate todevice 640. If before the third time device 610 joins yet another MANETor encounters other devices in radio range before joining the same MANETas device 640 or coming within radio range of device 640, it maybroadcast the certificate to the devices in the other MANET or to theother devices within radio range. If there are any devices in the otherMANET or within radio range that have a stored request to fetch device640's certificate up to the third time, any such device will store thecertificate in its local memory; alternately, all devices in the otherMANET or within radio range store the certificate in their local memoryup until the third time and upon joining the same MANET as device 640 orcoming within radio range of device 640, such a device could deliver thecertificate. Upon receiving its certificate, device 640 would sign itsnew certificate with its private key and broadcast the signature as anacknowledgement to other devices within radio range or within the MANET.When another device which had stored the request from device 640receives the acknowledgement, it can destroy the request in its localmemory and reallocate the freed memory to other purposes. If the thirdtime passes and a device that has stored a request from device 640 inits local memory, it can also destroy the request in its local memoryand reallocate the freed memory to other purposes.

It should be noted that depending upon the available memory in a device,the device might not be able to store requests and certificates for allrequesting devices. In this case the devices whose requests andcertificates would be stored could be selected randomly or pseudorandomly. An example of how this could be done would be to input arequesting device's address into a hash function and to honor therequest if the output of the hash function is within a predeterminedrange; the range could vary depending upon the amount of availablememory.

The above-described embodiments of the invention are intended to beexamples of the present invention and alterations and modifications maybe effected thereto, by those of skill in the art, without departingfrom the scope of the invention which is defined solely by the claimsappended hereto.

The invention claimed is:
 1. A method performed by a certificateauthority (CA) device for distributing certificates from the CA deviceto a first mobile device in a mobile ad hoc network (MANET), the MANETcomprising the first mobile device, a second mobile device, and one ormore access points (APs) for connecting to a communication network, theCA device capable of communicating with the one or more APs over thecommunication network, the method comprising: obtaining informationpertaining to the one or more APs of which the first mobile device andthe second mobile device are within range; determining the first mobiledevice was more recently in communication with the CA device compared tothe second mobile device; and sending a certificate to the one or moreAPs to enable the first mobile device to obtain the certificate whencoming into range with the one or more APs.
 2. The method of claim 1further comprising, prior to the CA device sending the certificate tothe one or more APs for the first mobile device, the CA device detectingthat bandwidth is limited for both the first mobile device and thesecond mobile device.
 3. The method of claim 1 further comprising, priorto the CA device sending the certificate to the one or more APs for thefirst mobile device, the CA device detecting that memory is limited forboth the first mobile device and the second mobile device.
 4. The methodof claim 1 wherein at least one of the first mobile device and thesecond mobile device are configured to communicate with other mobiledevices in the MANET.
 5. The method of claim 1 wherein the certificateinformation comprises a subset of full certificate information and thesubset includes changed timing information and a signature.
 6. Themethod of claim 1 wherein the MANET is established at periodicpredetermined times.
 7. The method of claim 6 wherein a time period forwhich the certificate is valid is correlated to the periodicpredetermined times.
 8. The method of claim 1 further comprising: if theCA device receives an acknowledgement that the first mobile device hasreceived the certificate before a certain time, the CA device does notdistribute the certificate for a time period subsequent to the certaintime.
 9. The method of claim 8 wherein the acknowledgement includes asignature of the certificate signed by the first mobile device's privatekey.
 10. A non-transitory computer readable medium comprising processorimplemented instructions executable by a certificate authority (CA)device for distributing certificates from the CA device to a firstmobile device in a mobile ad hoc network (MANET), the MANET comprisingthe first mobile device, a second mobile device, and one or more accesspoints (APs) for connecting to a communication network, the CA devicecapable of communicating with the one or more APs over the communicationnetwork, the instructions comprising: obtaining information pertainingto the one or more APs of which the first mobile device and the secondmobile device are within range; determining the first mobile device wasmore recently in communication with the CA device compared to the secondmobile device; and sending a certificate to the one or more APs toenable the first mobile device to obtain the certificate when cominginto range with the one or more APs.
 11. The non-transitory computerreadable medium of claim 10 further comprising, prior to the CA devicesending the certificate to the one or more APs for the first mobiledevice, the CA device detecting that bandwidth is limited for both thefirst mobile device and the second mobile device.
 12. The non-transitorycomputer readable medium of claim 10 further comprising, prior to the CAdevice sending the certificate to the one or more APs for the firstmobile device, the CA device detecting that memory is limited for boththe first mobile device and the second mobile device.
 13. Thenon-transitory computer readable medium of claim 10 wherein at least oneof the first mobile device and the second mobile device are configuredto communicate with other mobile devices in the MANET.
 14. Thenon-transitory computer readable medium of claim 10 wherein thecertificate information comprises a subset of full certificateinformation and the subset includes changed timing information and asignature.
 15. The non-transitory computer readable medium of claim 10wherein the MANET is established at periodic predetermined times. 16.The non-transitory computer readable medium of claim 15 wherein a timeperiod for which the certificate is valid is correlated to the periodicpredetermined times.
 17. The non-transitory computer readable medium ofclaim 10 further comprising: if the CA device receives anacknowledgement that the first mobile device has received thecertificate before a certain time, the CA device does not distribute thecertificate for a time period subsequent to the certain time.
 18. Thenon-transitory computer readable medium of claim 17 wherein theacknowledgement includes a signature of the certificate signed by thefirst mobile device's private key.
 19. A certificate authority (CA)device configured to communicate with one more access points (APs) overa communication network in a mobile ad hoc network (MANET), the MANETcomprising a first mobile device, a second mobile device, and the one ormore APs for connecting to the communication network, the CA devicecomprising a processor configured to distribute certificates from the CAdevice to the first mobile device, the processor configure to at least:obtain information pertaining to the one or more APs of which the firstmobile device and the second mobile device are within range; determinethe first mobile device was more recently in communication with the CAdevice compared to the second mobile device; and send a certificate tothe one or more APs to enable the first mobile device to obtain thecertificate when coming into range with the one or more APs.